The immune system usually takes 2 to 8 weeks to make antibodies against HIV (the average is 25 days). Although most HIV tests look for these antibodies, some look for the virus itself. The period after infection but before the test becomes positive is called the window period.
Deciding when to get tested therefore depends on when you may have been exposed and which test is used. See "What kind of tests are available and how do they work?" below.
A few people will have a longer window period, so if you get a negative test result in the first 3 months after possible exposure, you should get a repeat test after 3 months. Ninety-seven percent of people will develop antibodies in the first 3 months after they are infected. In very rare cases, it can take up to 6 months to develop antibodies to HIV.
Between the time you were possibly exposed and when you receive your test results:
Don’t have sex—or always use a condom if you do.
Don’t inject drugs—or always use clean equipment and don’t share needles or other equipment (works).